All-inorganic lead halide perovskites demonstrate improved thermal stability over the organic inorganic halide perovskites, but the cubic alpha-CsPbI3 with the most appropriate bandgap for light harvesting is not structurally stable at room temperature and spontaneously transforms into the undesired orthorhombic delta-CsPbI3. Here, we present a new member of black-phase thin films of all-inorganic perovskites for high-efficiency photovoltaics, the orthorhombic gamma-CsPbI3 thin films with intrinsic thermodynamic stability and ideal electronic structure. Exempt from introducing organic ligands or incorporating mixed cations/anions into the crystal lattice, we stabilize the gamma-CsPbI3 thin films by a simple solution process in which a small amount of H2O manipulates the size-dependent phase f ormation through a proton transfer reaction. Theoretical calculations coupled with experiments show that gamma-CsPbI3 with a lower surface free energy becomes thermodynamically preferred over delta-CsPbI3 at surface areas greater than 8600 m(2)/mol and exhibits comparable optoelectronic properties to alpha-CsPbI3. Consequently, gamma-CsPbI3-based solar cells display a highly reproducible efficiency of 11.3%, among the highest records for CsPbI3 thin-film solar cells, with robust stability in ambient atmosphere for months and continuous operating conditions for hours. Our study provides a novel and fundamental perspective to overcome the Achilles' heel of the inorganic lead iodide perovskite and opens it up for high-performance optoelectronic devices.